Blade Focus (new feat)
Due to the focus on a single blade, the master swordman to turn his strength into not only the sourse of his offensive power, but also his defensive, giving him the ability to use his strength mod instead of dexterity when calculating AC. (Only while wearing light or no armor)

Blade Mastery (new feat)
1: The swordmaster has achieved such precision he bypasses damage reduction.
2: In addition to 1, the swordmaster has been granted an extra attack per round at max BAB.
3: In addition to 1 and 2, the Swordmaster gains another extra attack at max BAB -5 and also gains +2 AC when using a single longsword.

Brutality/Guardian
Brutality increases all damage done by +5 when using a single longsword in addition to when inflicting a critical hit, he leaves the target bleeding for 5D6 for 1D4 rounds.
Guardian increases AC by +5 when using a single longsword in addition to increasing the AC of all companions by +3.

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It will need some serious balancing most likely, but none the less, would be nice to see.

This class is hyper OP. Not one but 2 extra attacks? On a high bab class? DR bypass? Up to 20d6 bonus damage on a crit or +5 (this is HUGE) AC coupled with a lvl10 bard permanent inspiration?
AC from strengh unless capped is way too good when you have classes giving you +10 strengh. Prepare for +20 AC on 50 strengh builds.

As I mentioned, needs some serious balancing, which is not my strong side, so consider the numbers placeholders until someone who can do proper balancing comes along.
Anyhow, though to justify some of the OPness of it, one do sacrifice all armor above light, sacrifices a real two hander, sacrifices the option to dual wield and sacrifices the option to use a shield.

Still, not to good at the balancing, just personally figured it could be an interesting character to play.

My design idea were basically to give one that "epic" feel of a legendary swordsman, relying purely on his ability with his longsword.

You can also achieve 'epic' feel by giving yourself +12 to all stats item in the toolset. :)

But considering that greatsword does 2d6 -> 7 damage, and longsword does 1d8 -> 4.5 damage, and you give the guy +5 damage, it's not like not having a 'true' twohanded weapon hurts you much, you still get 1.5 x str damage obviously. I did something like this with bard / rdd / bladesinger to spam twohanded longsword attacks. Also, NWN2 gives you MUCH better longswords than greatswords, on OC you can have +5 1d8 1d10 1d6 longsword for 18.5 damage, and greatsword +5 2d6 1d6 1d6 for 19 damage at best. So your build loses 0.5 damage, except that it gains the +5 so it actually gains +4.5.

Having new stats give you ac is always dangerous, especially so with strength since it's already your main stat (for example int for bladesinger would require you to take something you othervise may not, and even then there's some heavy limitations to it -> you can only get +10 which is same than motb shield). With this setup you'd just get 50 strength and have 'perfect' ac, perfect to-hit and perfect damage.

Losing out on the armor means only 1 or 2 ac points at best since you can just pick up a half decent dex item to fill out the dex requirement on the light armor.

Your hp is also 12, thats the very highest and is usually reserved to berserker types who lack ac. Fighters generally get 10 if they're pure fighters.

So long story short, you gain more ac, more damage, more attacks, more hp and can focus on single stat without much worry. It even has ref and will as strong saves which just happen to be the two giving trouble for fighters to patch up any weaknesses. I'm afraid this class would need more than 'rework', absolutely everything it has would need to be changed.

Also, like I said before, there already are plenty of weapon specialists out there, weapon master is pretty much just that. If you really must get no other options than longsword and no shield, then play bladesinger with a small dib on bard or something (you even get inspire bonuses for yourself), or if singing elves are not your fancy then just pick a regular fighter and CHOOSE to wield longsword in two hands, the damage is only slightly worse and you actually find better equipment in most cases since longsword is the most common thing around.

Ok, here's a gimped version of your master swordsman. It's STILL plenty strong and could probably use some work but this is really the least you could hope to go for: (even then, Kaedrin doesn't usually take requests unless the feature is something from an existing book, so still, I wouldn't get my hopes up.)

Blade Mastery (new feat)
Due to the focus on a single blade, the master swordman to turn his strength into not only the sourse of his offensive power, but also his defensive, giving him +1 dodge bonus to ac when using a single longsword (and nothing on the off-hand). This ability only works when master swordsman is wearing light armor or no armor.

Greater Blade Mastery (new feat)
When a 10th-level master swordsman makes a full attack with a longsword in one hand (and nothing in the other), he can make one extra attack in a round at his highest base attack bonus, but this attack and each other attack made in that round take a -2 penalty. This ability only works when master swordsman is wearing light armor or no armor.

Well I agree it looks more reasonable no doubt there, a few minor things I would change personally (yet again for the flavour, not the balance, since as before mentioned, never were my strong side), however yes it looks alot more balanced out now and still maintained the flavour that I intended mostly.
And honestly not keeping my hopes up, just decided to post it for the sake of it, well aware of the odds of it being made are close to nothing, but unless trying, there is no chance at all.

The longsword gets counted as twohanded weapon in this case after all, kinda like with bladesinger (it's not supposed to). So think of comparing it to a greatsword fighter and not a sword & shield character.

(You may argue that greatsword fighters are worthless, but that's a whole different issue anyway.)